In a traditional electrical utility grid model, the voltage and associated current are provided by large scale utility power plants running from expendable energy sources, such as coal, nuclear, natural gas and oil. However, modern electrical utility grids, in addition to receiving electrical power from traditional energy sources, can now also receive electrical power from multiple alternative and/or renewable energy sources, which may be directly or indirectly connected to the electrical grid. However, notwithstanding the particular types of energy sources (e.g., traditional, alternative, or renewable) providing the electrical power, all of those sources are normally coupled to the associated electrical grid through a grid connected unit, which converts and/or processes the generated electrical energy being sourced to the grid.
FIG. 1A depicts a conceptual power distribution system including a number of common electrical energy sources, which are either directly or indirectly connected to a conventional electrical grid 100 through a corresponding grid connected united. (In actual practice, the number and types of electrical energy sources connected to a given electrical grid can differ significantly. In addition, while the various energy sources shown in FIG. 1A are represented as consolidated entities, in actual practice, any of these energy sources could comprise multiple subsystems, which are physically and/or functionally distributed over a particular geographical area.)
Electrical grid 100 is controlled by a standard utility-scale control system 120 and is connected to a solar power plant 101, for example, through a central inverter, container solution, monitoring and supervision system, or plant controller. An electrical power to gaseous hydrogen (H2) generation plant 102 is also shown, which could connect to electrical grid 100 through an electrolysis rectifier. The system shown in FIG. 1A also includes a battery electrical storage system (BESS)/grid services system 103, which could connect to electrical grid 100 through a bidirectional inverter.
In the conceptual power distribution system of FIG. 1A, a wind park 104 connects to electrical grid 100, for example, through an uninterrupted power supply (UPS) system or an AC/DC converter. A local micro-grid 105 connects to electrical grid 100 through a subsystem such as a plant-scale intelligent power management system. Conventional electrical power plant 106 connects to electrical grid 100 through a UPS system, rectifier, inverter, or similar subsystem.
Electrical grid 100 is also shown connected to an office complex and data center 107, a set of residential users 108, an industrial complex 109, and an electromobility station 110 (e.g., electric filling station). Office and data center 107 could, for example, connect to electrical grid 100 through a UPS/DC infrastructure, while residential users 108 could connect to electrical grid 100 through a string inverter, intelligent substation, stabilizer, or the like. Industrial complex 109 connects to electrical grid 100, for example, through a power controller. A telecommunications hybrid power system 111, which could be either off-grid or grid-connected, could include a hybrid power generation system that integrates diesel, solar, and wind power generation capabilities.
Depending on the given observation point within the utility grid, a given electrical power source may provide electrical power in a range of low to high voltages, as a direct current (DC) or alternating current (AC), and/or in single or multiple phases. However, most parts of a typical electrical utility grid, as well as most conventional commercial/industrial systems, normally use three-phase alternating current (AC) power for power transmission and delivery. Therefore, some interface system or process is required for converting the corresponding forms of energy being generated by the various energy sources into a form and voltage compatible with the utility grid.
One such interface system is an electrical inverter that transforms the energy generated by a given energy source into sinusoidal AC power with a voltage compatible with the utility grid. For example, an inverter converts (or processes) energy from sources such as solar panels, wind turbines, steam turbines, DC battery plants, and the like, into a sinusoidal AC form and voltage compatible and consistent with the general AC electrical utility grid.
FIG. 1B is a high-level block diagram of a representative two-stage inverter unit 112 including a first stage 113 and a second (inverter power) stage 114. In this configuration, when the input is a DC signal, first stage 113 is a DC-DC converter and second stage 114 is a DC-AC converter. When the input is an AC signal, first stage 113 is a rectifier or an AC-DC converter and second stage 114 is a DC-AC converter. Galvanic isolation can be implemented in either of the first or second stages.
In the system shown in FIG. 1B, inverter power stage 114 comprises several blocks, including an inverter power train and a control scheme or function. Generally, the inverter power train switches power to transfer energy from its input to the inverter unit 112 output. The inverter power train can include one control input for duty cycle (e.g., pulse width modulation or PWM) control of the inverter power train or several control inputs for controlling three-phase AC outputs, depending on the power train complexity. State Space Vector control is one technique known in the art for computing the Duty Cycle (DTC) used to control an inverter power train, as might be used inverter power stage 114 of FIG. 1B. Space Vector Modulation control is described in numerous papers, such as Analog Devices Application Note AN21990-11 or the IEEE paper by Keliang Zhou and Danwei Wang entitled “Relationship Between Space Vector Modulation and Three Phase Carrier Based PWM: A Comprehensive Analysis.”
Assuming that the inverter unit output is connected to a pure resistive load, the inverter output voltage waveform is a sine wave. However, if the load is not a purely resistive, and hence the current and voltage are not in phase, the power must be delivered to the load as active and reactive parts. To process the Reactive Power (RP), the Active Power (AP) delivered to the load must be decreased and the inverter unit becomes less efficient. One solution to overcome the loss of the maximum AP available from the inverter unit, when RP must be processed, is to oversize the inverter unit.
In a Grid Connected Inverter (GCI), the GCI output voltage is essentially locked to the grid voltage, since the very low impedance of the grid makes it nearly impossible for the GCI to modify the grid voltage. Consequently, the current injected or pushed into the grid by the GCI is only determined by this very low impedance. In this case, the amplitude of the inverter output current is controlled with a PLL loop that increases or decreases the inverter output voltage and adjusts the phase shift of the current.
In a Grid Connected Inverter (GCI), the voltage magnitude is given by the grid. The impedance of the grid is so low that it is nearly impossible to modify the grid voltage magnitude. Consequently, the power delivered to the grid is controlled by injecting or pushing current into the grid with a current control loop.
A potentially dangerous condition known as islanding occurs on an electrical utility grid when the primary source supplying power to the grid or a branch of the grid is interrupted, but nevertheless one or more secondary sources continue to serve power to a section of the grid isolated from the primary source (an “island”). For example, a main power production facility may go offline or a failure may occur in the power transmission network, but wind turbines and solar panels may still continue to output power to a local branch of the power grid, so long as wind and sunlight are available.
The continued service of power to islands can create significant problems, including personnel hazards during grid maintenance and fault repair on lines that should otherwise not be energized, difficulties in re-energizing lines when the main power source is once again available, and difficulties in matching the secondary source to the grid on main power source restoration.
In view of the potentially serious problems that islanding can create, the operators of distributed power systems (e.g., electric utilities) normally require that some kind device or system be embedded in each distributed power source connected to the grid, which automatically disconnects that power source from the grid in the event of a main power disruption. This ability of the secondary (“island”) sources to automatically disconnect from the grid during main power disruption is commonly referred to as an “anti-islanding” function or feature. The anti-island function may be implemented, for example, in the GCI interfacing the secondary source with the grid.
In the case of a GCI interfacing a power grid with a solar, wind, or other electrical power source that continues to generate power after disruption of the main power source, it is mandatory to have an anti-islanding function to prevent energy flow onto the grid to avoid the problems discussed above.